A few layman hypothesis: * A more powerful venom will work faster and more reliably. * This is not only a very powerful, but also a very efficient venom. More

A few layman hypothesis: * A more powerful venom will work faster and more reliably. * This is not only a very powerful, but also a very efficient venom. More efficient venom reduces manufacturing costs (as only a few molecules are needed). * Chance probably have played a role here. Black widows hardly had any pressure to evolve a specific venom against humans. As our nerve system is based on the same basic molecules as every other land creature, but not the same structure, a venom that is good enough to kill a mouse may have a devastating (albeit non-fatal) effect on us. I'm already speculating, so I may be cruel: maybe the spider wants to paralyze the mouse, but keep him alive for a few days so the flesh stays fresh as they eat it.